This is completely legitimate Kotlin code. You can play with this code online (modify it and run in the browser) here.

How it works

Let's walk through the mechanisms of implementing type-safe builders in Kotlin. First of all we need to define the model we want to build, in this case we need to model HTML tags. It is easily done with a bunch of classes. For example, HTML is a class that describes the <html> tag, i.e. it defines children like <head> and <body>. (See its declaration below.)

Now, let's recall why we can say something like this in the code:

html {
// ...
}

html is actually a function call that takes a lambda expression as an argument. This function is defined as follows:

This function takes one parameter named init, which is itself a function. The type of the function is HTML.() -> Unit, which is a function type with receiver. This means that we need to pass an instance of type HTML (a receiver) to the function, and we can call members of that instance inside the function. The receiver can be accessed through the this keyword:

html {
this.head { /* ... */ }
this.body { /* ... */ }
}

(head and body are member functions of HTML.)

Now, this can be omitted, as usual, and we get something that looks very much like a builder already:

html {
head { /* ... */ }
body { /* ... */ }
}

So, what does this call do? Let's look at the body of html function as defined above. It creates a new instance of HTML, then it initializes it by calling the function that is passed as an argument (in our example this boils down to calling head and body on the HTML instance), and then it returns this instance. This is exactly what a builder should do.

The head and body functions in the HTML class are defined similarly to html. The only difference is that they add the built instances to the children collection of the enclosing HTML instance:

One other thing to be discussed here is how we add text to tag bodies. In the example above we say something like

html {
head {
title {+"XML encoding with Kotlin"}
}
// ...
}

So basically, we just put a string inside a tag body, but there is this little + in front of it, so it is a function call that invokes a prefix unaryPlus() operation. That operation is actually defined by an extension function unaryPlus() that is a member of the TagWithText abstract class (a parent of Title):

fun String.unaryPlus() {
children.add(TextElement(this))
}

So, what the prefix + does here is it wraps a string into an instance of TextElement and adds it to the children collection, so that it becomes a proper part of the tag tree.

All this is defined in a package com.example.html that is imported at the top of the builder example above. In the last section you can read through the full definition of this package.

Scope control: @DslMarker (since 1.1)

When using DSLs, one might have come across the problem that too many functions can be called in the context. We can call methods of every available implicit receiver inside a lambda and therefore get an inconsistent result, like the tag head inside another head:

html {
head {
head {} // should be forbidden
}
// ...
}

In this example only members of the nearest implicit receiver this@head must be available; head() is a member of the outer receiver this@html, so it must be illegal to call it.

To address this problem, in Kotlin 1.1 a special mechanism to control receiver scope was introduced.

To make the compiler start controlling scopes we only have to annotate the types of all receivers used in the DSL with the same marker annotation. For instance, for HTML Builders we declare an annotation @HTMLTagMarker:

@DslMarker
annotation class HtmlTagMarker

An annotation class is called a DSL marker if it is annotated with the @DslMarker annotation.

In our DSL all the tag classes extend the same superclass Tag. It's enough to annotate only the superclass with @HtmlTagMarker and after that the Kotlin compiler will treat all the inherited classes as annotated:

@HtmlTagMarker
abstract class Tag(val name: String) { ... }

We don't have to annotate the HTML or Head classes with @HtmlTagMarker because their superclass is already annotated:

class HTML() : Tag("html") { ... }
class Head() : Tag("head") { ... }

After we've added this annotation, the Kotlin compiler knows which implicit receivers are part of the same DSL and allows to call members of the nearest receivers only: